Passive chip-based droplet sorting

ABSTRACT

An apparatus for passive sorting of microdroplets including a main flow channel, a flow stream of microdroplets in the main flow channel wherein the microdroplets have substantially the same diameter and wherein the flow stream of microdroplets includes first microdroplets having a first degree of stiffness and second microdroplets having a second degree of stiffness wherein the second degree of stiffness is different than the first degree of stiffness. A second flow channel is connected to the main flow channel for the second microdroplets having a second degree of stiffness. A separator separates the second microdroplets having a second degree of stiffness from the first microdroplets and directs the second microdroplets having a second degree of stiffness into the second flow channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Division of Application No. 12/938,715filed Nov. 3, 2010, which claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 61/392,784 filed Oct. 13, 2010, thedisclosure of which is hereby incorporated by reference in its entiretyfor all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

BACKGROUND

1. Field of Endeavor

The present invention relates to droplet sorting and more particularlyto a system for passive sorting of microdroplets in a microfluidicsystem.

2. State of Technology

United States Published Patent Application No. 2008/0053205 fordroplet-based particle sorting provides the following state oftechnology information: “The present invention relates to droplet-basedparticle sorting. According to one embodiment, a droplet microactuatoris provided and comprises: (a) a suspension of particles; and (b)electrodes arranged for conducting droplet operations using dropletscomprising particles. According to another embodiment, a dropletmicroactuator is provided and comprises a droplet comprising a singleparticle in the droplet. According to yet another embodiment, a methodof transporting a particle is provided, wherein the method comprisesproviding a droplet comprising the particle and transporting the dropleton a droplet microactuator. According to a still further embodiment, amethod of providing a droplet comprising a single particle is provided,wherein the method comprises: (a) providing a droplet comprisingsuspension of particles; (b) dispensing a droplet from the droplet of(a) to provide a dispensed droplet; and (c) determining whether thedispensed droplet comprises a single particle and/or a desired particletype.”

U.S. Pat. No. 6,941,005 for monitoring and control of droplet sortingprovides the following state of technology information:

A. Flow Cell Chamber

One component of the system of the present invention, useful in themethod of the present invention is a cell sorter 100 which provides forsorting of cells or particles in a suspension which are contained in asample reservoir 102. The suspension is forced into a flow cell chamber104 where sheathing fluid from a sheathing fluid reservoir 106 surroundsthe sample as the sample enters the flow cell 104 from the sampletubing. This combination of sheath fluid and sample focuses thesuspension into a serialized order in the resulting stream. The flowcell 104 provides an analysis point where the focused sample intersectsa laser beam 108. The differences between the sample and sheathdescribed above are detected in the flow cell 104.

B. Droplet Generator

A droplet generator 114 is also included as a further component of thepresent invention. The droplet generator 114 perturbs the jet. By doingso, waves of undulations travel down the jet at the velocity of the jet.Preferably, a piezoelectric crystal is utilized to accomplishperturbation of the jet. The frequency of perturbation is set by afrequency generator (not shown), and may be varied as determined by oneof skill in the art. The drive amplitude is set by an amplifier (notshown). The jet forms as the stream is forced through the exit nozzle112 and breaks into droplets at the droplet generator drive frequency.

The elapsed time between the time the sample is detected by the laser108 in the flow cell 104 to the time that the stream is charged iscalled the delay time. The delay time must match the transit time of thedesired sample from the analysis point to the last attached streamsegment 116. The stream configuration must place the last attachedstream segment 116 in the same position as the sample to ensure goodsorting results. The stream configuration is manipulated in the presentinvention.

C. Detectors

Another component useful in the present invention is a detectionapparatus 118, which monitors the stream for specific particles andprovides a characterization of the contents of the stream. Typically,the suspension and the sheath fluid stream 110 typically flow into acuvette (not shown), which is illuminated by a light source 108.Preferably, the cuvette is present in a visualization chamber or portionof the flow cell 104. However, other chambers may be utilized to containthe suspension to be analyzed and may be selected by one of skill in theart. Suitable light sources include, without limitation, arc lamps,lasers, light bulbs, light emitting diodes (LED), among others.Typically, the light source 108 operates in a continuous mode.

D. Imaging Means

A further component of the present invention includes an imaging means38 (See FIGS. 6 through 12) to capture an image of the jet below thenozzle 112 according to the present invention. The imaging means can belocated in a variety of positions to capture one or more views of thejet, but is preferably located at the droplet forming region 32 (seeFIG. 1A) in the performance of this invention. A variety of imagingmeans are known in the art and can be utilized in the present inventionand include the imaging means described herein.

When the optical source or light field 120 illuminates the jet 110 belowthe nozzle 112, it strobes light at a frequency that is the same as thefrequency of the droplet generator 114, e.g., the piezo oscillator, ofthe flow cytometer 100. The light field 120 can strobe light at the samefrequency as the oscillator. In one embodiment, the imaging means isoperated at a frequency of about 0.6 to about 100 kilohertz; however thefrequency may be adjusted by one of skill in the art as needed.

E. Means for Generating the Numerical Standard and Sample Averages

Typically, the image of the jet (see FIG. 2B) contains noise. Typically,the noise present in the images is generated by the optical illuminationsource or light field 120. However other components of the instrumentcan generate undesirable nose in the images. By reducing or eliminatingthe noise, a more accurate representation of the jet can be obtained.Thus, in order to obtain an accurate image of the jet to be analyzed,the residual noise present in the images of the jet should be eliminatedor minimized. Typically, the noise is eliminated by subtracting abackground image. The present invention therefore provides for obtainingone or more background images that contain noise (see FIG. 2A). Thebackground images are typically obtained with no sheath or suspensionfluid flow present. In order to obtain an accurate background image,multiple images of the background without sheath or suspension fluidflow are obtained. Preferably, about 5 or more background images areobtained. More preferably, about 5 to about 40 background images areobtained. Even more preferably, about 5 to about 10 background imagesare obtained. Most preferably, about 10 background images are obtained.The multiple background images can optionally be averaged to obtain abackground image that is then used according to the invention.

Once the background image and image of the jet are obtained and the sameenhanced, the background image is subtracted from the image of the jetto remove noise. See FIG. 2C. If the background image and image of thejet are of the same size, no additional manipulations may be requiredprior to subtracting the background image from the image of the jet.Conventional and useful noise reduction techniques are also described inthe text of Tou and Gonzalez, cited above. If necessary, the images ofthe jet, before and after background subtraction, can be stored formanual or computer-assisted comparisons at a later date. Alternativelyand preferably, the images are instantly obtained and displayed on amonitor. An advantage of the present invention however includesoptimizing storage on a computer or optimizing the time required todisplay on a monitor and instantly processing the images according tothe following without storing the same for later use.

G. Adjusting Means

In order to maintain a stable sort, the system can be adjusted prior toand during sorting the particles of the suspension. If a significantdeviation of sample average from reference standard is detected, themethod involves an adjusting step, in which the processor is programmedto gives commands to the sample pressure regulator, the sheath pressureregulator and/or the piezo oscillator to force the deviant averages backto the reference standard representative of a sort in known, stable anddesirable condition. If unable to do so, the processor then issues othercommands to the sample pinch, sheath pinch, deflection voltage, streamcharge and the user interface to stop the sorting and alerts the user.

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides a system for passive sorting ofmicrodroplets in a microfluidic system. The system does not require ameasurement step as in other optic or electrically controlled sorters.Stiffness and viscosity of the droplets as the droplet contents changeand sorting is accomplished through changes in shear forces which arecontrolled by device bifurcation junction geometry and flow rate. Oneembodiment of the present invention provides an apparatus for passivesorting of microdroplets including a main flow channel, a flow stream ofmicrodroplets in the main flow channel wherein the microdroplets havesubstantially the same diameter and wherein the flow stream ofmicrodroplets includes first microdroplets having a first degree ofstiffness and second microdroplets having a second degree of stiffnesswherein the second degree of stiffness is different than the firstdegree of stiffness, a second flow channel connected to the main flowchannel for the second microdroplets having a second degree ofstiffness, and a separator for separating the second microdropletshaving a second degree of stiffness from the first microdroplets anddirecting the second microdroplets having a second degree of stiffnessinto the second flow channel. Another embodiment of the presentinvention provides a method of passive sorting of microdropletsincluding the steps of providing a main flow channel, providing a flowstream of microdroplets in the main flow channel wherein themicrodroplets have substantially the same diameter and wherein the flowstream of microdroplets includes first microdroplets having a firstdegree of stiffness and second microdroplets having a second degree ofstiffness wherein the second degree of stiffness is different than thefirst degree of stiffness, providing a second flow channel connected tothe main flow channel for the second microdroplets having a seconddegree of stiffness, and providing a separator for separating the secondmicrodroplets having a second degree of stiffness from the firstmicrodroplets and directing the second microdroplets having a seconddegree of stiffness into the second flow channel.

The present invention has use in biowarf are detection applicationsincluding identifying, detecting, and monitoring bio-threat agents thatcontain nucleic acid signatures, such as spores, bacteria, viruses etc.;in biomedical applications including tracking, identifying, andmonitoring outbreaks of infectious disease including emerging,previously unidentified and genetically engineered pathogens; inautomated processing, amplification, and detection of host or microbialand viral DNA or RNA in biological fluids for medical purposes; highthroughput genetic screening for drug discovery and novel therapeutics;genetic screening for oncology, disease, and personal genomics; compounddiscovery, proteomics, crystallography, and other research applications;and in forensic applications; automated processing, amplification, anddetection of DNA in biological fluids for forensic purposes; andexplosives detection and chemical processing.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIG. 1 is a flow chart illustrating components of a chip-based systemfor sorting droplets.

FIG. 2 is another flow chart illustrating components of system forsorting droplets.

FIG. 3 is a schematic of the chip-based system for sorting droplets ofFIG. 1.

FIG. 4 is another schematic illustrating the chip-based system forsorting droplets.

FIG. 5 is a chip-based method and system for protein crosslinking forviscous sorting.

FIG. 6 is a flow chart of macromolecular assembly.

FIG. 7 is a schematic of a system for protein crystallography sorting.

FIG. 8 is a schematic of a system for protein digestion sorting.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

Microfluidic devices are poised to revolutionize environmental,chemical, biological, medical, and pharmaceutical detectors anddiagnostics. “Microfluidic devices” loosely describes the new generationof instruments that mix, react, count, fractionate, detect, andcharacterize complex gaseous or liquid-solvated samples in amicro-electro-mechanical system (MEMS) circuit manufactured throughstandard semiconductor lithography techniques. These techniques allowmass production at low cost as compared to previous benchtop hardware.The applications for MEMS devices are numerous, and as diverse as theyare complex.

As sample volumes decrease, reagent costs plummet, reactions proceedfaster and more efficiently, and device customization is more easilyrealized. By reducing the reaction volume, detection of target moleculesoccurs faster through improved sensor signal to noise ratio over large,cumbersome systems. However, current MEMS fluidic systems may only bescratching the surface of their true performance limits as newtechniques multiply their sensitivity and effective throughput by ten, ahundred, or even a thousand times.

The present invention provides a method of enhancing a microfluidicdetector's limits and increasing its throughput by passively sortingdroplets that contain positive reaction products from those that don't,and allow microfluidic emulsion-based instrumentation systems tofunction well within the Poisson-regime, increasing the sensitivity downto single-copy levels and the accuracy of the instrument withoutsacrificing device throughput.

Microfluidic monodisperse droplet generators exist in the art and employvarious methods to produce continuous streams of droplets of identicalsize for use as chemical reactors or optical detectors in labon-chipapplications. These systems are very robust and efficient at generatingthe droplets, but they have no method for automatically sorting outdroplets whose fluidic properties change based on the chemical reactionsundergone in some or all of the manufactured droplets.

The system of the present invention provides a microfluidic architectureoptimized to use the applied velocity flow (and shear) fields to allow a“passive” sorting of droplets, where we use the term passive to signifythat no additional sorting steps or interventions are required such asoptical interrogation, applied electromagnetic force fields (such asdielectrophoresis, or DEP), and no applied pressure fields (fluidicpressure pulses or optical trapping pressures). The sorting method wedescribe here eliminates the expense, complexity, lowered throughput,and reliability issues associated with those previous “active” sortingtechniques.

The present invention provides an apparatus for passive sorting ofmicrodroplets including a main flow channel, a flow stream ofmicrodroplets in the main flow channel wherein the microdroplets havesubstantially the same diameter and wherein the flow stream ofmicrodroplets includes first microdroplets having a first degree ofstiffness and second microdroplets having a second degree of stiffnesswherein the second degree of stiffness is different than the firstdegree of stiffness. A second flow channel is connected to the main flowchannel for the second microdroplets having a second degree ofstiffness. A separator separates the second microdroplets having asecond degree of stiffness from the first microdroplets and directs thesecond microdroplets having a second degree of stiffness into the secondflow channel.

As the droplet reactors are typically employed, they contain solvatedchemicals intended to react in the droplets to serve a desiredanalytical purpose, such as chemical or biological species detection,polymer assembly, crystallography, nanoparticle synthesis, etc. All ofthese desired chemical reactions change the ratio of reactants toproducts internally in the droplets undergoing successful reactions. Fora large number of the current and future microfluidic uses of droplets,the change in the quantities and ratios of reactants and products,especially in large molecule assembly processes such as polymerizationand PCR-these changes alter the viscosity of the droplets and theirmechanical “stiffness” (the reciprocal of their compliance, “stiffness”has units of N/m and is essentially a spring constant.)

Designing a properly bifurcated flow channel junction that has beenoptimized for the monodisperse droplet size and carrier fluid will allowfor the selection of only the droplets that have a sufficient quantityof post-amplified or reacted material to change the droplet's viscosity,and hence its stiffness, which effects which path it takes at thesorting bifurcation. As droplets approach a bifurcation junction, theyare exposed to velocity gradients that result in shear gradients. Theseshear gradients apply a lateral force that can direct them to differentoutlets from where the flow lines would normally carry them. Whendroplets of different sizes but similar stiffness are exposed to theshear gradients, the method can separate out different size droplets-asshown in the prior art.

Applicants' method will exploit the fact that given droplets of similarsize but different stiffness, the “softer” droplets deform more than thestiffer ones reducing the lateral force exerted upon them, and hencetheir ability to move into the sort channel. For droplets that havelarge assembled molecular or polymer products, the large moleculesresult in a change in the droplet's overall stiffness. The overallchange in droplet stiffness varies roughly as a function of the ratio ofthe product to reactant concentration and the difference in the productto reactant molecular size. It is on this basis that Applicants'invention passively separates out the successful product producingdroplets simply based on channel geometry design and flow rate controlfor a given droplet stiffness and viscosity.

EXAMPLE(S)

In nucleic acid detection systems the starting concentration of targetednucleic acids is typically unknown, and varies over many orders ofmagnitude. Additionally, the samples are run dilute to ensure that nodroplets contain more than one starting copy to eliminate interferencefrom similar genomic templates. Therefore many droplets are generatedthat contain no genetic material to amplify. The droplets are then runthrough a Polymerase Chain Reaction (PCR) zone (FIG. 2). The proposedsorting system will only select the droplets that have a sufficientquantity of post-amplified nucleic acid material to change the droplet'sviscosity, and hence its stiffness, which effects which path it takes atthe sorting bifurcation. As droplets approach a bifurcation junction,they are exposed to velocity gradients that result in shear gradients.These shear gradients apply a lateral force that can direct them todifferent outlets from where the flow lines would normally carry them.When droplets of different sizes but similar stiffness are exposed tothe shear gradients, the method can separate out different sizedroplets. When droplets of similar sizes but different stiffness areexposed in these devices, the softer droplets deform more than thestiffer ones reducing the lateral force exerted upon it. For dropletsthat have PCR amplicons, the long chain DNA molecules result in a changein its overall stiffness. The overall change in droplet stiffness willvary as a function of the total number of DNA copies synthesized andtheir respective chain length. It is on this basis that Applicants'invention will passively separate out the PCR amplicons simply based onchannel geometry design and flow rate control.

FIG. 1 is a flow chart describing the items on a chip-based method andsystem for sorting droplets. The overall system is labeled 100.

In step one purified sample and reagents are carried by channel 12 tostep two where droplet formation and DNA isolation occur. Droplets ofuniform size are formed during step two and enter channel 16. Thechannel 16 is filed with a carrier fluid that does not mix with thedroplets therefore the droplets are carried along in channel 16 at spaceintervals. Some droplets contain DNA and some are empty. The dropletsproceed along the channel 16 to step three 18 where amplification takesplace. After amplification has taken place the droplets containing DNAwill have become stiffer than those droplets containing no DNA. Thedroplets now continue along the channel 16 to step four 20. In step fourthe droplets enter a bifurcated flow channel with branching arms 22 and26 of different sizes. The different sizes of the arms of the bifurcatedflow channel will cause the pressure to be greater in one of the arms 26than in the other arm 22. This pressure differential will cause thestiffer droplets to enter the arm 22 with less pressure and theessentially empty droplets being less stiff will be able to slightlydistort and pass through the arm 26 that has the higher pressure. Thestiffer droplets that have entered arm 22 will proceed to the dropletanalyzer 24. The empty droplets that have entered arm 26 will go todroplet waste 28.

FIG. 2 is another flow chart describing the items on a chip-based methodand system for sorting droplets. The overall system is labeled 200. Thissystem 200 is essentially the same as the system described in FIG. 1.The difference in the system 200 is that an additional step has beenadded between steps three and four of FIG. 1. The new step of system 200will become step three and the step four of FIG. 1 will now be calledstep five. This new step three of system 200 consists of a cooling stepwhich will further increase the stiffness of the droplets of interest.The droplets will then be processed as described in FIG. 1.

FIG. 3 is a schematic of the chip-based method and system for sortingdroplets of FIG. 1. The same reference numerals of items from FIG. 1will be used to identify the same items in the schematic of FIG. 3 withthe addition of items 32 the carrier fluid supply, 32 the carrier fluid,36 droplets containing material, 38 droplets with no material, 40amplified droplets and empty droplets. The overall schematic is labeled300. The system 300 here described starts with the purified sample andreagents entering the channel 12 and proceeding to 14 the dropletformation and DNA isolation step. The created droplets of uniform sizeare now in channel 16 which is filled with carrier fluid 34 suppliedfrom carrier fluid supply 32. The droplets some with material labeled 36and the droplets with no material labeled 38 now proceed along thechannel 16 to the amplification step 18. After the amplification stepthe droplets containing material, previously, labeled 36 will havebecome stiffer due to the increased material in the droplet, the dropletis now labeled 40 and the droplets containing no material are nowlabeled 42. The droplets now travel along channel 16 to the bifurcatedflow channel junction 20 where the stiffer droplets 40 will be inducedto enter arm 22 and the droplets 40 will proceed to droplet analyzer 24.The empty droplets 42 being less stiff than the droplets 40 will be ableto distort slightly and enter arm 26 and proceed to the droplet waste28. The droplets traveling along channel 16 and through the bifurcatedflow channel junction 20 are separated based upon the degree ofstiffness of the droplets. The separator includes an inlet to saidsecond flow channel 22 with the inlet having a diameter that is lessthan the diameter of the droplets 42. The main flow channel 16 has amain flow channel pressure and the second flow channel 26 has a secondflow channel pressure with said the flow channel 26 pressure being lessthan the main flow channel 16 pressure.

FIG. 4 is another schematic illustrating the chip-based method andsystem for sorting droplets. The overall system is labeled 400. Thesystem illustrated here is just like the system 300 previously describedexcept for the addition of a cooling station 30. This cooling station 30will add stiffness to the droplets that have been amplified thusenhancing the ability to sort full droplets from empty droplets.

Referring now to FIG. 5, a schematic of a chip-based method and systemfor macromoleculear assembly, specifically, protein crosslinking isshown. The overall system is designated generally by the referencenumeral 500. The same reference numerals of items used in FIG. 1 arealso used to identify the same items in the schematic of FIG. 5.Proteins produced by tissues and cells can be cross-linked usingsynthetic or natural cross-linking agents to form long macromolecularstructures.

The protein crosslinking system 500 begins with droplet containingsample 50 entering the channel 16 and proceeding to the macromoleculearassembly, protein crosslinking step 52. The channel 16 is filled with acarrier fluid. Note that empty droplets are also in channel 16.

In the macromolecular assembly step 52, proteins produced by tissues andcells are cross-linked using synthetic or natural cross-linking agentsto form long macromolecular structures. This produces the enhancedsample droplets 54. The enhanced sample droplets 54 and the emptydroplets labeled 56 enter channel 16. The enhanced sample droplets 54are stiffer than the empty droplets labeled 56 because the enhancedsample droplets 54 contain the enhanced sample.

The enhanced sample droplets 54 and the empty droplets 56 now proceedalong the channel 16 to the separation step. The droplets containingmaterial labeled 54 are stiffer than the empty droplets 56 due to thematerial in the droplets 54. The enhanced sample droplets 54 and theempty droplets 56 travel along channel 16 to the bifurcated flow channeljunction 20 where the stiffer droplets 54 will be induced to enter arm22. The stiffer droplets 54 will proceed to droplet analyzer 24 or otherprocessing. The empty droplets 56 being less stiff than the droplets 54will be able to distort slightly and enter arm 26 and proceed to thedroplet waste 28.

Referring now to FIG. 6, a flow chart of the macromolecular assemblyprotein crosslinking step52 of FIG. 5 is shown. Proteins produced bytissues and cells can be cross-linked using synthetic or naturalcross-linking agents to form long macromolecular structures. The fivejournal articles identified below described some examples of proteincrosslinking for viscous sorting. The five journal articles identifiedbelow are incorporated herein in their entirety for all purposes.

Article 1—“Photo-leucine and photo-methionine allow identification ofprotein-protein interactions in living cells;” by Suchanek, M.,Radzikowska, A. & Thiele, C.; Nat Meth 2, 261-268 (2005).

Article 2—“Metal-Mediated Self-Assembly of Protein Superstructures:Influence of Secondary Interactions on Protein Oligomerization andAggregatio,” by Salgado, E. N., Lewis, R. A., Faraone-Mennella, J. &Tezcan, F. A.; Journal of the American Chemical Society 130, 6082-6084(2008).

Article 3—“The cross-linking of proteins with glutaraldehyde and its usefor the preaparation of immunoadsorbernts;” by Avrameas, S. & Ternynck,T.; Immunochemistry 6, 53-66 (1969).

Article 4—“Inducing and Sensing Protein-Protein Interactions in LivingCells by Selective Cross-linking;” by Lemercier, G., Gendreizig, S.,Kindermann, M. & Johnsson, K.; Angewandte Chemie International Edition46, 4281-4284 (2007).

Article 5—“Self-assembly and cross-linking of bionanoparticles atliquid-liquid interfaces;” by Russell, J. T. et al.; AngewandteChemie-International Edition 44, 2420-2426 (2005).

Protein-protein interactions are the key to organizing cellularprocesses in space and time. The only direct way to identify suchinteractions in their cellular environment is by photo-cross-linking.The macromolecular assembly step 52 produces the enhanced sampledroplets 54.

Referring now to FIG. 7, a schematic of a system for proteincrystallography sorting is shown. The overall system is designatedgenerally by the reference numeral 700. The same reference numerals ofitems from FIG. 1 are used to identify the same items in the schematicof FIG. 7.

A droplet generator 64 produces droplets 50 containing reagents forproducing crystals. For example, the droplet generator 64 can be adroplet generator that uses a variety of reagents for producing crystalsfor X-ray crystallography. The droplets 50 need to incubate in order todetermine which droplets and reagent mix will successfully producecrystals.

The droplets 50 produced by the droplet generator 64 proceed to theincubate step 60. In the incubation step 60, some of the droplets haveproduced droplets with crystals 62 and some of the droplets 56 have notproduced crystals. The droplets containing crystals 62 are stiffer thanthe empty droplets 56 due to the crystal material in the droplets 62.The droplets 62 and 56 travel along channel 16 to the bifurcated flowchannel junction 20 where the stiffer droplets 62 will be induced toenter arm 22. The stiffer droplets 62 will proceed to droplet analyzer24 or other processing. The empty droplets 56 being less stiff than thedroplets 54 will be able to distort slightly and enter arm 26 andproceed to the droplet waste 28.

Referring now to FIG. 8, a schematic of a system for protein digestionsorting is shown. The overall system is designated generally by thereference numeral 800. The same reference numerals of items from FIG. 1are used to identify the same items in the schematic of FIG. 8.

The system 800 starts with droplet generator 80 producing droplets whichenter the channel 16 and proceeding to the protein digestion step 82.The droplets entering the protein digestion step 82 contain cells,virions, or other degradable structures. The droplet are processed instep 80 which produces droplets 84 and 86. The droplets 84 contain morestiffness-enhancing material such as large intact cells or otherdigestible material and are stiffer than the droplets 86 that contain nocells or completely broken down material because the large and intactmaterial in the droplets 84 makes them stiffer.

The droplets 84 and 86 travel along channel 16 to the bifurcated flowchannel junction 20. The bifurcated flow channel junction 20 acts as asorter to separate droplets 84 from droplets 86. The stiffer droplets 84will enter arm 22 and proceed to droplet analyzer 24 or otherprocessing. For example, if the aims of the sorter channel junction 20are to accumulate the digestion enzymes that work on the cells,accumulate the digestion products from within the broken cells, orharvest desired cellular contents if the cells are being used asbio-reactors, then the stiffer droplets 84 will correspond to intactcells, whereas droplets with the broken down cells will be less stiffand be the desired product sent to droplet analyzer 28 or otherprocessing.

The droplets 86 being less stiff than the droplets 84 will be able todistort slightly and enter arm 26. The droplets 86 will proceed todroplet analyzer 28 or other processing. For example, if the aims of thesorter channel junction 20 are to collect the cells that don't getdigested (i.e. test for specific antigens, antibodies, or identificationof the bacteria/cell type) these undigested cell droplets 84 will go toanalyzer 24 or other processing.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A method of passive sorting of microdroplets, comprising the stepsof: providing a main flow channel, providing a flow stream ofmicrodroplets in said main flow channel wherein said microdroplets havesubstantially the same diameter and wherein said flow stream ofmicrodroplets includes first microdroplets having a first degree ofstiffness and second microdroplets having a second degree of stiffnesswherein said second degree of stiffness is less than said first degreeof stiffness, providing a second flow channel connected to said mainflow channel for said second microdroplets having a second degree ofstiffness, and providing a separator for separating said secondmicrodroplets having a second degree of stiffness from said firstmicrodroplets and directing said second microdroplets having a seconddegree of stiffness into said second flow channel.
 2. The method ofpassive sorting of microdroplets of claim 1 wherein said step ofproviding a separator for separating said second microdroplets having asecond degree of stiffness from said first microdroplets and directingsaid second microdroplets having a second degree of stiffness into saidsecond flow channel includes providing a bifurcation junction forseparating said second microdroplets having a second degree of stiffnessfrom said first microdroplets and directing said second microdropletshaving a second degree of stiffness into said second flow channel. 3.The method of passive sorting of microdroplets of claim 1 wherein saidstep of providing a separator for separating said second microdropletshaving a second degree of stiffness from said first microdroplets anddirecting said second microdroplets having a second degree of stiffnessinto said second flow channel includes providing a restriction forseparating said second microdroplets having a second degree of stiffnessfrom said first microdroplets and directing said second microdropletshaving a second degree of stiffness into said second flow channel. 4.The method of passive sorting of microdroplets of claim 1 wherein saidstep of providing a separator for separating said second microdropletshaving a second degree of stiffness from said first microdroplets anddirecting said second microdroplets having a second degree of stiffnessinto said second flow channel includes providing a restriction in saidsecond flow channel wherein said restriction has a diameter less thansaid diameter of said microdroplets for separating said secondmicrodroplets having a second degree of stiffness from said firstmicrodroplets and directing said second microdroplets having a seconddegree of stiffness into said second flow channel.